1. Field of the Invention
The subject invention is directed to anti-fouling polyethylene compositions and particularly anti-fouling ultrahigh molecular weight polyethylene “AF-UHMWPE” compositions which are resistant to fouling in aqueous, particularly marine environments, and to the use of such compositions.
2. Background Art
Items exposed to aqueous environments, particularly those of reservoirs, lakes or streams, and in particular salt water, tend to accumulate a large growth of biological organisms on their surface. This growth is particularly problematic in warmer climates. Ocean-going ships are but one example. In days gone by, clipper ships and wooden steamships were covered with copper plates below the waterline to minimize growth of molds, algae, barnacles, and the like. However, even with such copper plates, vessels still had to be periodically scraped below the waterline. In the salty environment, the copper plating life was only a few years at best.
Modern ships of steel have the same problems. Growth of organisms greatly increases the power requirements necessary to drive the ship through the water. Top speed is limited as well. Anti-fouling paints have replaced copper plates as the anti-fouling medium. These paints, in general, contain large amounts of copper and tin compounds. Some paints are designed to have a very slick surface such that organisms will be swept away at speed. However, such coatings are not feasible on large ships, whose maximum speed is relatively slow. Another common anti-fouling paint is one which “sloughs off” over time, continually exposing a fresh surface, and hence “fresh” anti-fouling agents. All these anti-fouling paints containing heavy metals, such as copper, are being reviewed for their potential harm to the environment.
Seachests are another example of an item exposed to marine environments. Seachests are large “coffin-like” bodies located at the bottom of a ship's hull, through which water may enter or leave the bilge. Seachests are described in U.S. Pat. No. 6,904,858 whose disclosure in this regard is incorporated herein by reference. Further non-limiting examples of items where aquatic growth should be minimized include floating docks, buoys, piers, water holding tanks, and the like.
It is known to incorporate biocides into polymers to minimize microbial growth. For example, Feldhahn et al. U.S. published application 2008/0032119 A1 discloses medical articles such as vaporizers whose surface is textured and hydrophobed by pressing hydrophobic particles into a polymer containing a biocide, and Kohler et al. U.S. published application 2004/0082492 A1 discusses incorporating biocides into polymer shapes used as packings in heat transfer apparatus used in heating and air conditioning (HVAC) equipment. Copeland U.S. published application 2007/0160766 A1 discloses coatings prepared by dissolving a hydrocarbon resin, a thermoplastic polymer, and a biocide into an organic solvent.
UHMWPE is a unique polymer with exceptional strength and lubricity characteristics. UHMWPE is distinguished from other polyethylene polymers such as high density high molecular weight polyethylene in that the ultra high molecular weight restricts processing by conventional thermoplastic polymer processing techniques such as extrusion, roto forming, and injection molding. UHMWPE resin is supplied as fine particles which must be consolidated by what is essentially a high pressure sintering at elevated temperature. Both batch processing processes as well as ram extrusion processes have been used. Net-shape processing may also be used, for example for the ski runners of snowmobiles.
In batch and net-shape processes, the UHMWPE powder is introduced into a very thick-walled mold and consolidated at high pressure and at elevated temperature. Temperatures of from 160° C. to 280° C. may be used, for instance. At higher temperatures, decomposition and oxidation processes may start to occur. The crystalline melt temperature of UHMWPE is about 130° C. to 135° C. See for instance, U.S. Pat. No. 5,453,234, which is incorporated herein by reference.
In the ram extrusion process, UHMWPE powder is introduced into a chamber and rammed at high pressure through a heated zone, and through a die of the proper geometry. This process has been used for many years to produce geometrically symmetrical moldings such as cylindrical extrusions such as rods and tubes in large cross-sectional sizes, and asymmetrical moldings of more limited size such as flats, channels, etc. Uneven shrinkage, distortion, and other problems have prevented the formation of sheet-type products of significant width until recently, as evidenced by U.S. published applications 2008/0020182, 2008/0018022, 2008/0018011, 2008/0018026 which are incorporated herein by reference. Processing aids such as stearic acid or stearate salts are sometimes added, and extrusion may take place after displacement of air by nitrogen to minimize oxidation. However, the choice of additives which can be employed with UHMWPE is more limited than with other polymers, because not only must these survive the high temperature and pressure of sintering, but must also not induce or catalyze oxidation reactions. In ram extrusion processes, the additives must also not increase the friction between the UHMWPE and the heated die.
UHMWPE, like other polymers, is also subject to fouling. In marine environments, a distinction may be made between soft fouling, e.g. growth of algae and like organisms, and hard fouling such as by barnacles, mollusks, bryozoans, etc. In many applications, resistance to both soft and hard fouling is desirably present. It would be desirous to provide for articles of UHMWPE which take advantage of the strength properties of the latter and are processable by standard UHMWPE processes, while being resistant to both hard and soft fouling as well as improving the ease in which fouling can be removed.